The present invention relates to a wiping material for a vacuum chamber for use in the preparation of a silicon wafer and a process for the production thereof. More particularly, the present invention relates to a wiping material for a vacuum chamber made of a polyurethane foam which can exhibit sufficient moisture retention and wiping properties and a process which can fairly prepare such a wiping material.
A silicon wafer indispensable for the production of computer chips or the like is produced by subjecting a desired substrate to vacuum metallization with a metal such as tungsten, aluminum and copper. In this process, a vacuum chamber is used. It is known that a metal which has been ionized during the vacuum metallization step is deposited as a sludge on the inner wall of the vacuum chamber to grow in laminar form with time. The sludge thus deposited on the inner wall of the vacuum chamber causes unevenness in the vacuum metallization on the substrate and the drop of efficiency in vacuum metallization as well as gives adverse effects such as contamination by impurities. Thus, the sludge must be fully removed depending on the circumstances of vacuum metallization.
The foregoing full removal operation involves (1) a step of scraping the surface of the foregoing solidified sludge with an abrasive pad or the like, (2) a step of wiping out a sludge left unremoved with a wiping material impregnated with a detergent, and (3) a final step of wiping the inner wall of the vacuum chamber with the foregoing wiping material until there is no effect on the vacuum metallization on the substrate to finish the inner wall of the vacuum chamber. As the wiping material to be used at the steps (2) and (3) there is used a material free of silicone, which has adverse effects on the vacuum metallization on the substrate, and containing little lint, such as nonwoven cellulose cloth.
However, when the foregoing nonwoven cloth is used to conduct removal of sludge, a large amount of nonwoven cloth is required because the foregoing deposit is a sludge and wiping is conducted with a detergent. By way of example, about 30 sheets of nonwoven cloth are used per one operation. This operation is conducted about 15 times. Accordingly, the nonwoven cloth is used in an amount as large as about 450 sheets per vacuum chamber. As the foregoing nonwoven cellulose cloth there may be used a relatively inexpensive product made of regenerated material. However, since severe requirements must be met to wipe out the sludge, the amount of nonwoven cloth to be used must be increased anyway. Further, since the nonwoven cloth can be hardly re-used, it is essentially disadvantageous in that the running cost increases in all.
A nonwoven cloth is essentially a cloth and thus can be easily clogged. Thus, a nonwoven cloth has not so good capability of scraping and wiping a sludge. Further, since a nonwoven cloth is an aggregate of fibers, it is said disadvantageous in that it inevitably produces lint, though in a small amount. Moreover, since an individual nonwoven cloth is not thick enough, it is said disadvantageous in that it can hardly be handled manually during cleaning operation.
Thus, it can be proposed that various spongy foams be used as wiping material instead of the foregoing nonwoven cloth. These foams are easily available in a thickness suitable for gripping by the operator and thus have a good handleability. Further, these foams can be easily freed of sludge and detergent when washed after operation. These foams are also advantageous in that they produce no lint. However, these foams are obtained by foaming with a silicone-based surface active agent as a foam stabilizer. Accordingly, these foams are greatly disadvantageous in that when used to wipe the interior of the vacuum chamber, silicone remains on the inner wall of the vacuum chamber, giving impurities which contaminate the resulting silicon wafer and other products.
The present invention has been worked out to give good solution to the foregoing problems with the use of prior art wiping materials for a vacuum chamber.
It is therefore an object of the present invention to provide a wiping material for a vacuum chamber having sufficient sludge wiping properties and water retention attained by easy control over cell diameter and use of a polyurethane foam which has been foamed free from a silicone-based surface active agent and a process for the production thereof.
The foregoing object of the invention will become apparent from the following detailed description and examples.
The object of the invention is accomplished by the following aspects of the invention.
(1) A wiping material made of a polyurethane foam for a vacuum chamber for use in the preparation of a silicon wafer, wherein the polyurethane foam is free of silicone and has an open-cell structure comprising cells the diameter of which are controlled to fall within the range of from 500 xcexcm to 3,000 xcexcm.
(2) A process for the production of a wiping material made of a polyurethane foam for a vacuum chamber for use in the preparation of a silicon wafer which comprises previously mixing at least a polyol and tolylene diisocyanate to prepare an isocyanate-terminated prepolymer, mixing the isocyanate-terminated prepolymer with a non-silicone-based surface active agent having an HLB value of from 9 to 20, various additives and water, and then allowing the mixture to undergo reaction and foaming in a manner controlled such that the resulting wiping material has cells with a diameter of from 500 xcexcm to 3,000 xcexcm and is free of silicone.
(3) A process for the production of a wiping material made of a polyurethane foam for a vacuum chamber for use in the preparation of a silicon wafer which comprises mixing at least a polyol, tolylene diisocyanate, a non-silicone-based surface active agent, various additives and water, and then allowing the mixture to undergo reaction and foaming in a manner controlled such that the resulting wiping material has cells with a diameter of from 500 xcexcm to 3,000 xcexcm and is free of silicone.